1. Field of the Invention
This invention relates to a method and apparatus for acquiring a radiation image signal. This invention particularly relates to a method and apparatus for acquiring a radiation image signal, wherein light emitted by a stimulable phosphor sheet, on which a radiation image has been stored, when the stimulable phosphor sheet is exposed to stimulating rays, is received with a light receiving device comprising photodiodes containing silicon as a principal constituent.
2. Description of the Related Art
Radiation image recording and reproducing systems comprising radiation image recording apparatuses, radiation image read-out apparatuses, and the like, in which stimulable phosphors are utilized, have heretofore been known as computed radiography (CR) systems. With the CR systems, a radiation image of an object, such as a human body, is recorded as a latent image on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. In this manner, the image signal representing the radiation image of the object is acquired.
As the radiation image read-out apparatuses for the CR systems, there have heretofore been known radiation image read-out apparatuses, wherein red stimulating rays having been produced by a semiconductor laser are irradiated to a stimulable phosphor sheet, on which a radiation image has been stored, light emitted by the stimulable phosphor sheet is detected with a photomultiplier via a stimulating ray cut-off filter for transmitting the light emitted by the stimulable phosphor sheet and blocking the red light, and a radiation image signal representing the radiation image having been stored on the stimulable phosphor sheet is thereby acquired.
Ordinarily, besides the red light, the red stimulating rays produced by the semiconductor laser also contain infrared rays. The red light is filtered out by the stimulating ray cut-off filter, and the infrared rays pass through the stimulating ray cut-off filter. However, the photomultiplier has little sensitivity with respect to the infrared rays. Therefore, in cases where the light, which is emitted by the stimulable phosphor sheet exposed to the red stimulating rays and has wavelengths falling within the visible light wavelength region, is detected with the photomultiplier, the infrared rays contained in the red stimulating rays are not detected.
Recently, there is a strong demand for radiation image read-out apparatuses having a reduced size, and research has been conducted to develop radiation image read-out apparatuses, in which a charge coupled device image sensor (a CCD image sensor) having a small size are utilized in lieu of the photomultiplier, the light emitted by the stimulable phosphor sheet is received with the CCD image sensor, and the radiation image signal representing the radiation image having been stored on the stimulable phosphor sheet is thereby acquired.
The CCD image sensor is the light receiving device comprising photodiodes containing silicon as the principal constituent. The CCD image sensor has a high sensitivity with respect to not only the visible light, but also the infrared rays. Therefore, in cases where the infrared rays contained in the stimulating rays having been produced by the semiconductor laser pass through the stimulating ray cut-off filter and impinge upon the CCD image sensor, the infrared rays are detected and constitute noise in the acquired radiation image signal. Accordingly, it is desired that an infrared ray cut-off filter capable of blocking the infrared rays is inserted to a position between the stimulating ray source and the stimulable phosphor sheet or between the stimulable phosphor sheet and the CCD image sensor, and the infrared rays are thus prevented from being detected.
However, the filter, which is to be inserted to the position between the stimulating ray source and the stimulable phosphor sheet and has sufficient characteristics for transmitting the red light necessary for the stimulation of the stimulable phosphor sheet and blocking the infrared rays, requires a markedly high production cost. Also, the filter, which is to be inserted to the position between the stimulable phosphor sheet and the CCD image sensor and has sufficient characteristics for transmitting the light to be detected and with wavelengths of the visible light wavelength region and for blocking the infrared rays, requires a markedly high production cost. Therefore, the problems occur in that it is not always possible to utilize the aforesaid filter in the radiation image read-out apparatuses.
The problems described above ordinarily occur in cases where the light emitted by the stimulable phosphor sheet is detected with the CCD image sensor and in cases where the light emitted by the stimulable phosphor sheet is detected with the light receiving device comprising the photodiodes containing silicon as the principal constituent.